Render turns black when finished
It sounds like the Use Nodes check mark in the compositer is checked, but the image is not connected to the output.

To fix the above issue you can:


If you are not using the compositer on your image, uncheck the Compositing check mark in the Properties window.

If you do want to use the compositer, then you will need these two nodes at a minimum; the Render Layers and the Composite nodes.